It is generally assumed that the effective service life of spark plugs on any given engine is limited by the maximum voltage demand required to break down the spark gap between the electrodes and the ability of the ignition system used to deliver the required voltage to the spark plug. This invention is based on the discovery that for any given spark plug design for use in any given engine, the spark plug life is additionally limited by the maximum size of the electrode gap. This limitation is independent of the capability of the ignition system to deliver the required voltage. The service life of spark plugs is greatly extended by the means taught in this patent.
Experience has shown that the actual end of spark plug service life is often limited at a breakdown voltage well below the actual output capacity of the ignition system. This observed phenomena has generally been attributed to the limited dielectric capability of the components connecting the high voltage source to the spark plug. This problem has been so pervasive that the engine community has even created a specific term for this problem, "spark plug flashover". In many cases where the end of life was occurring at low voltages relative to the capability of the ignition system, the dielectric limit of the connecting system may have been correctly identified as the root cause of the failure of the spark plug to initiate combustion within the cylinder of the engine. If an effort to eliminate this problem, spark plug manufacturers have increased external ceramic insulator lengths and ignition suppliers have developed better leads and wiring approaches. Some engine manufacturers have even gone to a coil on plug approach to reduce the distance traveled by the high voltage external to the spark plug to an absolute minimum. With all of these improvements and in spite of the fact that the external dielectric limit has been greatly extended by the use of these better wiring and insulation techniques, the proper operation of the spark ignited engine is still often limited to an in-cylinder voltage demand well below the ignition system capability. These observed engine misfire conditions are often incorrectly attributed to a lack of an electrical discharge event or to the assumed discharge through some path external to the spark plug, for example, a defective plug, wire, ignition coil or the like.
In reality, under engine misfire conditions with worn spark plugs, many times the electrical discharge does occur inside the combustion chamber although not between the spark plug electrodes where intended. In the case of the current spark plug designs, the cause of the engine misfire is often a surface spark discharge of the plug inside the power cylinder of the engine at the center electrode down the center electrode ceramic insulator to the grounded shell. This occurs on spark plugs not intended for surface discharge operation. This unintentional surface discharge is a most significant problem for two distinct reasons.
In the first case, even if the surface discharge occurs more or less normally with a spark duration roughly equivalent to the normal arc, the energy transfer to the air/fuel mixture is still terribly inefficient due to the decreased surface area of the spark in contact with the mixture and the loss of localized heating of the mixture to the cooler insulator surface, and far more likely to experience quenching of the infant flame kernel due to the loss of self-sustaining combustion heat to the insulator surface. This quenching phenomena is known to those skilled in the art of spark ignited engines. Surface gap spark plugs are specifically designed to overcome this problem.
The second phenomena has been to the best of my knowledge previously unidentified. Not only is the infant flame kernel subject to quenching by this surface contact, but also infant electrical sparks (arcing events) suffer from a similar problem. In the period immediately following the breakdown event, the arc is often observed to be momentarily interrupted (see FIG. 1) when the breakdown occurs at relatively high voltages (25 kV or more). This appears to occur regardless of the discharge path. However, dependent upon the actual discharge path, the results of the next event in the sequence are vastly different (see FIG. 2). When the arc is established normally through the gaseous media between the intended electrodes, the arc re-establishes itself almost immediately and with a very low second breakdown requirement (5 kV or less, see FIG. 2, trace A). When this occurs, the total energy transferred is not measurably different than a single spark event and has been treated by those skilled in the art as though it were a single event. When the discharge path is across the surface of the solid insulating material, the arc also seems to interrupt immediately after being established, however the breakdown voltage required to re-strike the arc is significantly higher than the previous case (see FIG. 2, trace B). This is because unlike the breakdown through the air/fuel mixture, which is rich in highly charged ions, the gas molecules in the boundary layer near the insulator are a poor donor of electrons and they are in short supply after the initial breakdown event. As a result, in the surface discharge case, the voltage demand of the re-strike may be nearly equal to or even greater than the original surface spark event (see FIG. 2, trace B greater than 20 kV). After the first surface discharge event, a large portion of the ignition system energy has been expended and the arc may not re-strike at all. Even if a re-strike or "arc continuation" does occur (see FIG. 5) due to this much higher additional or second breakdown requirement, a spark event of extremely short duration occurs and inadequate energy is transferred to the mixture to initiate normal combustion.
FIGS. 4, 5 and 6 show the impact on a typical used spark plug of an electrode erosion of only 0.004 inch upon the tendency of the spark to discharge via a surface route instead of between the intended electrodes. These figures show that the surface discharge occurs at an even lower voltage (less than 25 kV) than with a new plug and it also occurs with a much greater frequency. This is an important factor in the effective plug life since as the erosion occurs, the average voltage required for proper engine operation continuously increases.